Utilities and other entities operate distribution systems for water, gas, and electricity to deliver these resources to various consumers connected to the distribution system, with a meter at each point the resource is removed from system to measure the consumer's usage. Many metering systems utilize wireless communications modules operatively connected to the meter itself (which may be referred to as endpoints, nodes, or the like, and which hereinafter will be referred to generically as a meter interface unit, or MIU), such that the MIU reports the meter reading electronically to a communications network. The network may include a mobile communications device that collects the transmitted readings as a utility worker drives within range of the meters, or stationary receivers or collectors designed to receive messages from a designated set of meters within a certain range. Various network topologies and technologies exist in the prior art for transmitting meter readings from an MIU at the meter either through an intermediary device (whether stationary or mobile, including MIU's acting as an intermediary device in a mesh network) and on to a central data collection or processing unit. In this disclosure, the term “collector” shall refer to any device that receives transmissions of meter readings from an MIU in an automated meter reading (AMR) system, in any network architecture or topology. In AMR systems, the communications module at the meter might transmit a reading on a predetermined interval (a “bubble up” system), or the module might respond to a command to report a meter reading from the central host or a nearby receiver or collector. The details of such networks are known and understood by those of ordinary skill in the art and will not be discussed in further detail here. In any case, such communication systems offer the ability for rapid transmission of meter readings from the meter itself back to the central host computer for compilation and analysis.
Typically, an MIU transmits a data packet containing one or more meter readings on pseudorandom intervals. The data packet typically contains the most recent reading or readings, which roll off the packet based on age as more recent readings become available. For example, assume a packet contains data fields for three readings, and readings are taken every fifteen minutes, but transmitted every five minutes. A given set of three readings (the most recent reading, the reading taken fifteen minutes before the most recent reading, and the reading taken thirty minutes before the most recent reading) is therefore transmitted three times before a new reading is taken and the oldest reading is dropped from the packet to make room for the new reading. In this scenario, each reading is transmitted three times before being put in a new position and nine times before being replaced by a newer reading.
Data can be lost, however, if there is some problem preventing a successful transmission or receipt of the reading before the reading completes its progression through the available data fields in the packet as newer readings are made. These problems include, for example, weather conditions, environmental conditions, an outage in the receiving network, or other interference. Water meters in particular are often contained in pits, below ground level, as are the MIUs. If there is heavy rainfall, the pit may fill with water, which interferes with the transmission capabilities of the antenna. Similarly, a delivery driver or other person may park a vehicle immediately over or next to the meter location, which could block transmission of the meter readings. While these weather, environmental, or network conditions are temporary, if they were to last more than an hour in the example above, meter readings would be lost.
As referenced above, MIUs (especially those in water, gas, or chemical systems) often operate on batteries, which are intended to last for many years. MIUs also are programmed to take certain actions, such as readings and transmissions, upon certain time intervals, and it may be important to know exactly when a reading was made. MIUs therefore contain clock or timing circuits. While accurate timing is desirable, a highly precise clock that does not drift over the intended life of the MIU is relatively expensive and may consume more power than a less expensive clock. Given that there are often very large numbers of MIUs in a distribution system (tens of thousands), the incremental cost of a highly precise clock is multiplied by the large volume of units required. Apart from considerations of cost and power consumption, the clocks in MIUs may drift or lose accuracy because of the wide temperature ranges to which they are exposed, humidity levels, and aging of components. Preventing such drift could require regular testing and maintenance of the MIUs or better components, both of which add costs to operation of the system. On the other hand, there typically are far fewer collectors than MIUs, because hundreds or even thousands of MIUs may be served by a single collector. Collectors are usually connected to a fixed power source. Because they are substantially fewer in number, collectors are more readily maintained and upgraded than MIUs. It would be advantageous to shift the cost, power, and maintenance requirements of a highly precise clock from the MIU to the collector.
Thus, there exists a need for an AMR system in which the time that meter readings were made can be determined accurately without incurring the full cost of a highly precise clock in the MIU. There also exists a need for an AMR system in which meter readings will not be lost because of temporary obstructions that interfere with the transmission or receipt of meter readings.